Project Summary B cells are important for the generation of protective immunity and function at multiple discrete stages. The unique role that B cells play as antibody-producing cells is appreciated, however the other functions of B cells as antigen presenting cells, lymphoid tissue organizers and cytokine-producing effector or regulatory cells are much less understood and rarely studied. Recent work from our lab showed that cytokine producing effector B cells play crucial roles in the development of protective humoral and cellular immunity to infectious agents. We therefore hypothesized that there are specialized populations of B cells that have the ability to secrete cytokines and regulate immune responses. To address this hypothesis, we developed an in vitro culture system using primary mouse B cell to generate different classes of cytokine producing effector B cells. We characterized these in vitro generated cells and found markers that we subsequently used to identify the cytokine producing B cell subpopulations in vivo. For example, using markers identified in our in vitro cultures, we can now identify IFN&#947;producing B cell effectors in vivo and showed that these B cells are specifically induced in response to influenza infection and are elevated in number in some mouse models of autoimmunity. Thus, this approach has allowed us for the first time to visualize B cell effectors in vivo and has opened the door to functional and mechanistic studies that will reveal the roles for these cells in infectious and autoimmune disease. Despite the significant progress that we have made in our studies of mouse effector B cells, little is known about their human B cell counterparts. In fact, only very rudimentary studies have been conducted to date with human B cells. From these limited studies it is clear that human B cells have the potential to make cytokines under some stimulation conditions. However, these studies have been conducted with bulk peripheral blood B cells, thus the effector human B cells subsets remain to be identified. The goals of this proposal are to (i) to develop an in vitro culture system to phenotypically and functionally characterize human effector B cells and (ii) To use the information that we have obtained from the in vitro cultures to identify effector B cells in samples from healthy and autoimmune individuals.